A capacitive video disc system has been described by Clemens in U.S. Pat. No. 3,842,194. The grooved video disc of Clemens had a conductive metal coating to provide the conductivity required for capacitive pickup and a thin layer of dielectric material thereover. An electrode on the playback stylus completed the capacitor.
Presently, video discs are prepared from a conductive plastic material such as disclosed by Martin et al in U.S. Pat. 4,228,050 issued Oct. 14, 1980, entitled "Conductive Molding Composition." The molding compositions described by Martin et al include a molding resin such as a vinyl chloride homopolymer or copolymer and a sufficient amount of a particulate conductive material, such as carbon black, so that the disc record can provide capacitive readout. The conductive capacitive information disc may be utilized with either a grooved or non-grooved information disc record system.
A conductive molding composition such as disclosed by Martin et al contains, in addition to sufficient conductive particles to provide capacitive readout without a conductive coating, additives such as flow modifiers, lubricants, stabilizers and the like. Such compositions containing large quantities of additives must be blended in a high intensity mixer such as a Henschel, Welex or Papenmeier type mixer to obtain the requisite homogeneity. It is preferred to blend certain of the powdered components of such molding compositions in dry form. This dry blending can be carried out at various stages in the preparation of the final conductive molding composition. The dry blending may also be carried out on the total composition or combinations of certain ingredients thereof. In most instances, dry blending will be carried out on a mixture of the polyvinylchloride resin and the conductive particles, preferably with one or more of the other powder ingredients of the formulation. It has been found that, regardless of what stage in the preparation of such a conductive molding composition dry blending is utilized, there is produced a buildup of powder around the walls of the mixer. Such "rings" of powder are predominately carbon black which has a tendency to separate from the blend due to its low bulk density.
The buildup of powder "rings" inside the mixer is, of course, disadvantageous in that such powder is not pure and is usually discarded because the cost of the analysis which would be required in order to recycle it is prohibitive. This separation of powder during dry blending will also change the proportions in the final mixture to a degree. This may be critical, for example, if a substantial quantity of an additive present in the formulation in minor amount is found in the "rings". Any deleterious effect on the homogeneity of the final composition which might be caused by the formation of powder "rings" in the mixer during dry blending can be potentially serious since the playback characteristics of the video disc are, in large measure, dependent on the disc having uniform electrical properties. In accordance with this invention, there is provided a means whereby the homogeneity of the conductive molding composition is enhanced and the powder buildup on the mixer virtually eliminated.